Abstract:
We present high‐resolution ionosonde observations at Allahabad (25.43°N, 81.84°E) near
anomaly crest location in India along with GPS TEC to investigate the possible signatures of co‐seismic disturbances (CIDs) in the ionosphere during Mw7.8 Nepal Earthquake that occurred at 06:11:26 UT on 25 April 2015. The epicenter of the earthquake (28.147°N, 84.708°E) was located at ~77 km north‐west of Kathmandu with a shallow depth of 15 km. The mechanical shock disturbances produce Rayleigh waves and Rayleigh wave induced acoustic gravity waves (AGWs) during earthquake which propagate into the ionosphere and produced plasma density fluctuations due to collisions. The manifestation of these waves produces ‘wiggles' or ‘cusps' in the ionograms known as Multiple‐Cusp Signatures (MCS). The detection of multiple cusps in the ionograms at Allahabad indicates the altitude structure of Rayleigh wave and it's induced AGWs. To confirm co‐seismic signatures in ionosonde further, we scaled the ionograms at iso‐density and iso‐heights along with foF2 (MHz) and h'F (km). The periodogram and wavelet analysis suggest
the presence of acoustic gravity waves and gravity waves with frequencies in the range of 0.7‐1.5 mHz over the altitude range of 180‐250 km. The foF2 variations show wave like oscillations having frequency of 0.5 mHz after the onset of earthquake. The GPS TEC also show signatures of co‐seismic perturbations having frequencies in the range of 2‐4 mHz and are propagated to southward with a speed of 0.5‐1.0 km/s. These results suggest that there exists a good coupling between seismic activity and ionized density perturbations after Nepal earthquake.
